With lithium niobate (LNB) devices such as directional couplers and Mach Zehnder (MZ) interferometers, there exists the problem that in order to achieve a given level of extinction, switching or modulation, continually greater electrode potentials are required throughout the operating life of the device. This phenomenon is known as voltage induced drift. Ultimately a limit is reached where the driving electronics are supplying their maximum potential and it is no longer possible to achieve the desired performance. Alternatively the device may fail catastrophically with electrode breakdown caused by the high applied potential. Moreover, there is a further disadvantage of using high electrode potentials with directional couplers, in that the extinction ratio is impaired relative to that attainable with lower electrode potentials.
Clearly devices subject to such drift are unsuitable for any long term systems applications such as telecommunications or optical signal processing, because there is no long term certainty that any particular device output corresponds to a certain applied potential.
One method which has been suggested as a means of overcoming the problem of voltage induced drift in electro-optic devices for long term systems use is to divert part of the device's optical output to a detector in order to monitor the drift so that the bias voltage can be varied to trace it. The disadvantages of this hypothetical arrangement are twofold: first, the difficulty of monitoring part of the optical output and deriving the required bias level: second, the drift tends to continue at a near linear rate, consequently higher and higher bias levels are still required.